IP phone systems that convert voices, etc. into packet data, and transmit this packet data via an IP (Internet Protocol) net in real time are being used widely. With such an IP phone system, a plurality of IP phone terminals is accommodated by a switch of a local network realized by LAN (Local Area Network) or the like. In addition, at the same time, global networks such as a public network and the internet are connected with a local network via the router device of the local network.
Then, by the router device performing address conversion, etc., not only is communication between IP phone terminals of a local network being realized via a switch, but also communication between an IP phone terminal of a global network with an IP phone terminal of a local network is being realized.
In addition, with such an IP phone system, a NAT (Network Address Translator) function is built into the router device in order for the router device to perform address conversion, etc. Herein, NAT function is a function of converting the local address in sending to a global address and then sending as a sending source IP address, and then converting the global address of a destination IP address provided in receiving into a local address.
An example of such an IP phone system including a router with built-in NAT function is described in Patent Document 1. Patent Document 1 describes a system in which an SIP phone device based on SIP (Session Initiation Protocol) and a router are connected to LAN, and a telephone under the SIP phone device calls an SIP telephone connected to an ISP network, via the router and SIP phone device.
Further, Patent Document 1 has the object of solving a problem in that a small-scale telephone switching device such as a button telephone set (key telephone) that accommodates a plurality of phones and connects to a network cannot connect to a plurality of ISP networks with one telephone switching device.
In order to solve this problem, the technology described in Patent Document 1 configures so that one telephone device is able to mutually access each router via LAN by providing an external line port to each of a plurality of routers interfacing with corresponding ISP networks, respectively. The telephone device can thereby access with a corresponding ISP network via each router, in a call connection between SIP telephones via an external line from an extension telephone. For this reason, it is possible to communicate with a plurality of ISP networks, and thus possible to solve the aforementioned problem.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2008-172552